1. Field of the Invention
The invention relates in general to a structure of an overlay mark and its dosimetry application. In particular, this invention relates to a structure of an overlay mark that can prevent damage caused by chemical mechanical polishing process due to the design of the inventive structure surpasses the conventional structure and corresponds to the dosimeters of X,Y directions, thus, enhances measurement accuracy and analysis method of the overlay error.
2. Description of the Related Art
In addition to the control of critical dimension (CD), factors for a successfull photolithography process on a wafer include alignment accuracy (AA). Therefore, the measurement of accuracy, that is, the measurement of overlay error is crucial to the semiconductor fabrication process. An overlay mark is used as a tool for measuring overlay error and to determine whether the photoresist pattern is precisely aligned with the previous wafer layer on a wafer after a photolithography process.
FIG. 1 is a top view of a wafer that illustrates positions of conventional overlay marks.
In FIG. 1, after the wafer 100 is formed, the wafer 100 is sawed along scribe lines 104 into a plurality of chips or dies 102. Normally, the overlay marks 106 are located on the scribe lines 104 at the four corners of the edge of each chip 102 to measure whether the photoresist pattern is aligned with the previous wafer layer in the fabrication process.
FIG. 2 is a cross-sectional view cutting along the line I-Ixe2x80x2 of FIG. 1. A part of the structure of the overlay mark and the neighboring chip is shown. The overlay mark is applied to an interconnection fabrication process, which is further described as follows.
In FIG. 2, a metal layer 202 is formed in the substrate 200. A dielectric layer 205 with a via hole 206 and a trench 207 therein is formed on the substrate 200. The via hole 206 has a narrow width. A metal layer 204 is formed over the dielectric layer 205 to completely fill the via hole 206, but to cover only a surface portion of the trench 207. A chemical mechanical polishing process is performed to remove the metal layer 204 that is formed out of the via hole 206 and the trench 207. The dielectric layer 205 is thus exposed that is the dielectric layer 205 is used as a stop layer to form a plug within the via hole 206. A metal layer 208 is formed on the dielectric layer 205 to fill the trench 207. Because the trench 207 is sufficiently wide, therefore, the metal layer 208 over the trench 207 has a recess 211. Due to the recesses 211 on the metal layer 208, scribe lines are formed on the metal layer 208.
A number of processes such as photoresist coating, an exposed process and a photolithgraphic process are carried out to form a patterned photoresist layer 210 on the metal layer 208. Therefore, a region for forming conductive wires in the metal layer 208 is exposed. A photoresist pattern 210a is formed between the scribe lines and is combined with the recess 211 as an overlay mark 212 for an accurate measurement.
FIG. 3 shows a top view of a conventional structure of an overlay mark.
Referring to FIGS. 2 and 3, FIG. 3 is a top view of an overlay mark 212 formed by a combination of the recess 211 and the photoresist pattern 210a. A conventional overlay mark 212 includes an outer mark 302 and an inner mark 304. The outer mark 302 comprises four recesses 211 as shown in FIG. 2, while the inner mark 304 comprises the photoresist patterns 210a that constructs another rectangle. The outer mark 302 embraces the inner mark 304. The overlay mark 212 is located on the scribe lines at four corners of each chip to measure whether the photoresist pattern is precisely aligned with the previous layer.
FIG. 4 illustrates a cross section taken along a cutting line II-IIxe2x80x2 of FIG. 3.
Referring to FIGS. 3 and 4, the recesses 211 in FIG. 4 correspond to the outer mark 302 in FIG. 3, and the photoresist pattern corresponds to the inner mark 304.
FIG. 5 shows the signal waveform of the overlay mark as shown in FIG. 4.
Referring to FIGS. 4 and 5, the peak signals of the recesses 211 in FIG. 4 are denoted as 502a and 502b in FIG. 5, and the peak signals of the photoresist pattern 210a are denoted as 506a and 506b in FIG. 5. Using the conventional overlay mark to measure the alignment accuracy, the peak signals 502a, 502b of the recesses are read first. A mean value 504 of the peak signals 502a and 502b is obtained. A mean value 508 of the peak signals 506a and 506b is then obtained after being read. The difference between the mean values 504 and 508 is calculated as the overlay error. If the overlay error is larger the acceptable deviation, the alignment between the photoresist pattern and the wafer does not reach the required accuracy. Consequently, the photoresist has to be removed, and the photolithography process has to be repeated until the overlay error is no larger than the acceptable error.
However, after chemical mechanical polishing, the quality of the conventional outer mark of the overlay mark is affected or even damaged due to the factors such as a polishing rate deviation, a slurry corrosion, a density of patterns on the wafer and the polishing deviation between the wafer center and edge. Further, the grain of the metal layer is an important factor for affecting on the accuracy of the overlay mark because if the size of the grain is too big, it will affect the measuring signal, leading to a poor measurement of signal profile of the peak signal is obtained. The measurement result is thus seriously affected because the distance between the outer marks 302 (that is, the recesses 211) of the conventional overlay mark is too long. That is, because the distribution of the recesses 211 is too scattered, and because the scattered structure, the damage caused by chemical mechanical polishing is not withstood. The chemical mechanical polishing performed after formation of the metal via, especially the copper damascene, plays an important role for the subsequent process due to the integrity of the overlay mark. This is because the problems of stability, slurry anti-corrosion and diffusion cause a more serious effect to copper than other metal such as tungsten.
FIG. 6 shows the method for measuring the overlay error using the conventional overlay mark.
Referring to FIG. 6, while measuring the overlay error using the conventional overlay mark, the X-directional deviation is measured along a straight line 310 in X-direction of the overlay mark 212. A Y-directional deviation is further measured along a straight line 312 in the Y-direction of the overlay mark 212. When all the overlay marks 212, which are being set in the scribe lines are measured using this method, whether the photoresist pattern and the previous wafer layer on the chip are precisely aligned can be calculated according to the X- and Y-directional deviations.
However, one conventional overlay mark 212 can only measure one X- and one Y-directional deviations. If the outer mark 302 is damaged during the chemical mechanical polishing process, the X- or Y-directional deviation cannot be measured, and the alignment accuracy cannot be obtained correctly.
The invention provides a structure and a fabrication method of an overlay mark. The probability of damaging the overlay mark by chemical mechanical polishing process is reduced.
The invention further provides an overlay mark structure, and the measure and analysis method thereof to enhance the accuracy for measuring the overlay error.
The structure of the overlay mark provided by the invention includes an outer mark and an inner mark. The outer mark encloses a closed cross area which comprises two central axes. The inner mark is made of four strip patterns arranged in the central axes and extends outwardly towards four directions from a central part of the closed cross area.
The invention further provides a method of measuring overlay error. An overlay mark is provided. The overlay mark comprises an outer mark and an inner mark. The outer mark encloses a closed cross area that comprises two central axes. The inner mark is arranged in the central axes and includes two X-directional strip patterns and two Y-directional strip patterns extending outwardly from a central part of the cross area. An X-/Y-directional deviation along the X-/Y-direction is measured. An X-/Y-directional straight line cuts across a Y-/X-directional extension area of the closed cross area and one of the Y-/X-directional strip patterns.
The invention also provides a method for analyzing the overlay error factors. An overlay mark with an outer mark and an inner mark is firstly provided. The outer mark encloses a closed cross-area with two central axes. The inner mark includes two strip patterns arranged in an X-direction and two strip patterns arranged in a Y-direction. A first X-/Y-directional deviation is measured along a first X-/Y-directional straight line that cuts across one of Y-/X-directional extension areas of the closed cross area and one of the Y-/X-directional strip patterns. A second X-/Y-directional deviation is measured along a second X-/Y-directional straight line that cuts across the other Y-/X-directional extension area of the closed cross-area and the other Y-/X-directional strip pattern. A plurality of overlay error factors can be obtained from first X-/Y-directional deviations and the second X-/Y-directional deviations (four deviations). The overlay error factors include a displacement error and a reticle rotation (RR) error inducing by a position of a photoresist layer.
The overlay mark provided by the invention includes a reinforced structure that effectively withstands the chemical mechanical polishing process that can damage the outer mark during interconnection fabrication process. Therefore, the probability of damage caused by chemical mechanical polish is greatly reduced.
The inner mark of the overlay mark of the present invention is defined by the photoresist layer. In general, the signal transmission is very good, therefore an unstable problem due to measure two directions on one side only to form one mark shall not be concerned. In the contrary, measuring the directions of the two X,Y points of one single set provides twice information than the convention method, and it can increase the accuracy of the calculation and the sample.
In addition, while using the overlay mark and measurement method of the invention, the accuracy of measurement of the overlay error is enhanced.
Furthermore, only one overlay mark is required to obtain the reticle rotation error in the invention.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.